To achieve labor saving and higher speed in testing, sample processing systems that automatically perform pre-processing and conveyance of samples to be loaded in automatic analyzers are used in clinical laboratory settings in which qualitative and quantitative analyses of biological samples such as blood and urine are conducted. Such sample processing systems perform various types of processing for samples stored in, for example, sample vessels and loaded in the systems. Patent document 1 (JP-2011-33395-A), for example, discloses a technique relating to an automatic unplugging apparatus. In the technique, out of a plurality of test tubes held in an upright posture in one row in a test tube rack, the automatic unplugging apparatus simultaneously clamps plug bodies of every other test tube and moves upwardly to thereby remove the plug bodies. The automatic unplugging apparatus then simultaneously clamps plug bodies of the rest of the test tubes and moves upwardly to thereby remove the plug bodies from the rest of the test tubes.